The investigation also found that Pegatron violated labour laws by hiring underage workers, breaching contracts and demanding staff work excessive hours. Apple has promised to investigate its supplier.

An undercover reporter described working on the plastic cover, explaining: "Today's work is to paste protective film on the iPhone's plastic back cover to prevent it from being scratched on assembly lines.

"This iPhone model with a plastic cover will soon be released on the market by Apple. The task is pretty easy, and I was able to work independently after a five-minute instruction from a veteran employee.

"It took around a minute to paste protective film on one rear cover. The new cell phone has not yet been put into mass production, so quantity is not as important. This makes our job more slow-paced than in departments that have begun mass production schedules."